I just noticed something on my Rigol DS2072A. Not a big issue but kind of strange.

When ever I adjust the trigger level in DC and HF-Reject coupling mode there is an orange cursor/line on the display for about 2-seconds showing the trigger level (very handy). However when I use AC or LF-Reject coupling the orange cursor is not there??? Seems like it should be there.

Speaking of triggers has anyone been able to successfully use the EXT TRIG input? I have been able to use it to read a frequency from an external signal generator (using the Measure/Counter/EXT menu settings) but for some reason (probably user error) I haven't been able to use it a way that triggers a signal.

The manual says:

External trigger input:External trigger source can be used to connect external trigger signal to the EXT TRIG channel when both of the two channels are sampling data. The trigger signal (such as external clock and signal of the circuit to be tested) will be connected to EXT trigger source via the [EXT TRIG] connector. You can set the trigger condition within the range of trigger level (-4 V to +4 V).

Maybe that means both channels need to be actively sampling data at the same time? Even so, I tried that but haven't been able to figure out how to cause the trigger to occur. (I tried by taking a signal out of a Tek analog scope's channel output and triggering the signal on the Tek scope but it didn't trigger the signals on the Rigol.) Any suggestions? Thx

With AC coupling, the trigger level will vary with duty cycle so having a numeric value for the trigger level may be misleading. Some oscilloscopes, both analog and digital storage, disable the trigger level readout when AC coupling is used and some do not. The DC level information may only be lost when the trigger coupling itself is AC but not when only the vertical input coupling is AC.

There are other trigger level design decisions like whether vertical positioning affects trigger level which can lead to one oscilloscope design operating differently than another. It gets even more complicated in some oscilloscopes where the vertical position control also acts as an offset control. In others, they are separate functions and the separate vertical position control may or may not affect the trigger level.

David, thanks. Both scope were set for DC coupling. My thought was that when I triggered the analog scope the trigger output should have then triggered the signals on one or both of the Rigol channels - but maybe not? Thx, EF

With AC coupling, the trigger level will vary with duty cycle so having a numeric value for the trigger level may be misleading.

That's a good explanation - thanks.

I'm still not sure why there would be a difference between handling trigger cursor display between HF and LF rejection. I'm not even sure if they are AC or DC coupled.

HF reject means a low pass filter is inserted into the trigger path so it is DC coupled. LF reject means that a high pass filter is inserted into the trigger path so it is AC coupled. Neither will allow an absolute trigger level measurement but for different reasons.

A Tektronix 2247A is a good real world example. AC or DC coupling of the vertical input does not affect the trigger level display because the oscilloscope measures the levels after that point and the trigger level just has to match what is on the CRT but AC, LF reject, and HF reject trigger coupling disables the trigger level display because either the DC information is lost or in the case of HF reject coupling, the loss of high frequencies causes the trigger to be late.

A Tektronix 2232 on the other hand always displays the trigger level if a vertical input is the trigger source but in LF reject trigger coupling, the trigger level is only accurate in a relative sense and does not represent the absolute trigger level as shown on the CRT. In HF reject mode, the extra delay caused by loss of the high frequencies causes a similar error.